Modern digital projection systems use spatial light modulators, also referred to as light valves or digital light processors. This technology has enabled higher levels of picture quality and realism. There is a growing demand to create large panoramas with digital projection technologies in many economic and industrial sectors. These include entertainment, video-gaming, simulation training, military operations, advertising and business applications, all of which require or benefit from projection of large, high-quality images. With demand for display of large, high-resolution continuous images, system designs must address numerous complex projection problems. For example, image displays on large cylindrical and dome-shaped screens often require projection of a series of overlapping images. Each projected image is generated from a separate projector in a multi-channel display system to build extended horizontal or vertical fields of view, e.g., up to 360°. However, sustaining high image quality in the transitions between adjoining image tiles can be difficult in view of optical effects and brightness issues resulting from pixel overlap.
To enhance realism it is desirable to render transitions between individual image tiles unnoticeable, i.e., seamless. Achieving a seamless transition between adjoining image tiles depends in part on alignment of pixel data from different projectors on the screen. Additional treatments are commonly applied to regions where beams from different projectors contain overlapping image data. Otherwise, abrupt changes in brightness would result from projection of duplicate pixel data onto the same region of a screen. The term “soft edge matching” was coined in the 1990's and involved overlapping these images to transition the end of one channel with the start its adjacent channel.
A variety of treatments have been applied to mitigate such noticeable changes in screen brightness, but these have performance limitations or other disadvantages. With multiple projectors beaming the same pixel information toward a region of overlap, treatments for image blending include software-driven electronic adjustments and hardware-based techniques. Software-driven electronic adjustments, referred to as “software blending”, have proven to be an effective alternative to system designs which closely align image tiles without permitting any overlap of the pixel information in adjoining tiles. As systems incorporate larger or more complex projection geometries, it becomes more difficult to connect non-overlapping image projections while retaining a high quality, seamless appearance, e.g., without creating disruptions at the interfaces of tile edges being joined into a continuous image.
All blending techniques attempt to adjust the spatial distribution of screen brightness to approach levels which would otherwise result from a single image projection. However, some of these blending techniques may compromise image resolution and may be less satisfactory under conditions of low screen brightness levels.
Brightness adjustments effected with software blending can create seamless transitions between image tiles by adjusting the screen brightness within individual tile projections, i.e., by digitally varying projector output levels as a function of pixel position on the screen. Software blending adjusts the contribution to a screen brightness level from each of multiple projectors across a blend zone. This reduces the net brightness level in a region of overlap from a level which would otherwise include additive effects of duplicate image information on the same screen area. In a well-known implementation of software blending across a region of image tile overlap, the brightness level contribution from one projector spatially varies along one direction from a maximum value to a minimum value while the brightness level contribution from an adjacent projector spatially varies along the same direction from a minimum value to a maximum value.
Software blending can generate acceptable brightness levels to provide a seamless transition across a region of image tile overlap, provided there is sufficient dynamic range to modify the levels to a visually acceptable brightness. At relatively high brightness levels software blending methods can adjust screen brightness levels in a region of tile overlap that would be equivalent to the levels produced by one projector beam. However, software blending is less effective at low light levels (e.g., night training) despite off-state pixels which, in theory, do not illuminate on-screen. The projector lamp produces excess light bouncing off of reflective surfaces and various internal lenses within the projector, and low level residual light is still emitted from the projector's objective lens even with no active pixels on-screen. Systems using digital mirrored modulating devices (e.g., TI's DLP, SONY's SXRD) do not provide a true black light level (i.e., no light output) as the lowest level of screen brightness. Rather, at the darkest level (e.g., a digital zero), systems comprising digital light processors project some light. In regions of tile overlap, when a brightness level is supposed to be at or near digital zero, the brightness level which results after adjustment with software blending can remain noticeably too high. There is limited dynamic range available to optimally reduce brightness resulting from duplicate pixel data. As the brightness levels output by the projectors approach a minimum digital value, it becomes impossible to electronically reduce screen brightness due to overlapping pixels by, for example, thirty to eighty percent. Consequently, at such very low light levels, visibly evident brightness artifact are not removed by software blending. When image tile overlap regions display night scenes containing important but relatively dim image information (i.e., at or near the lowest digital values), features inherent to the projector display technology limit the precision of brightness adjustment. Under these conditions, software blending methods cannot create the desired seamless transition between image tiles.
In lieu of software blending, two distinct types of hardware-based optical device designs and methods have been used to adjust regions of image tile overlap in tiled arrays: optical blending and optical blocking. These may be used in place of or in addition to software blending methods. Both optical blending and optical blocking are useful alternatives under low light conditions. Optical blending is accomplished with blend plates while optical blocking is performed with blocking mask plates. Although similarities exist between a few of the components in optical blending systems and optical blocking systems, optical blend plates do not and cannot perform the functions of optical blocking mask plates.
The primary function of an optical blend plate is to blend or mix and to smooth abrupt changes in brightness levels across regions of tile overlap. Blend plates do not completely block off light along each side of an image tile transition line. Rather, they retain overlapping pixel data while reducing overall light levels in tile transition zones to reduce noticeability of transitions between adjoining edge tiles. Generally, optical blend plate designs form a class of devices that obscure transitions between image tiles by scattering some of the light present in the projection beams. For each pair of overlapping projection beams, a pair of blend plates creates a blend zone on the projection screen. This is accomplished by insertion of edge profiles in front of portions of the two overlapping projection beams. Through absorption or scattering, blend plate edge portions remove or redistribute light before the beams impinge on blend regions on screen. Scattering is effected by incorporating light mixing features along the edge profiles. The light mixing features enhance reflection or diffraction in the portions of the beam adjoining the transition between image tiles. Some of the scattered light may impinge on the projection screen.
Optical blending can create sufficient diffuse light or scattering by edge diffraction techniques to spatially modify brightness levels within a tile overlap region and thereby provide a seamless transition zone. The light may be absorbed or diffused in a limited portion of an image projection by placing a series of closely spaced surfaces in part of the path of the projection beam. The resulting scattered light reduces the spatial gradient in brightness level across the transition zone to render the tile transitions less noticeable. On the other hand, the scattering process could introduce significant noise with possible loss of pixel resolution. These effects must be limited to avoid obscuring image details in low light level scenes and to avoid noticeable degradation in image quality.
The closely spaced features of light mixing edge profiles used for optical blending may be regular patterns (e.g., saw tooth patterns) or spaced-apart appendages having relatively small feature sizes (e.g., formed with fine brush hairs or comb-like teeth). These features may extend from one or more larger members of a blend plate for insertion into an image projection path. The blend plate light-mixing edge profiles do not and cannot operate as optical blocking masks. This is because such regular patterns and appendages do not transfer a shadow contour consistent with necessary blocking patterns. The features do not conform to provide a line of transition that removes duplicity of pixel projections in adjoining image tiles. Rather, projection of such small, spaced-apart features is only suitable for blurring or reducing light levels based on scattering or absorption. With light mixing appendages designed to primarily scatter light, these features are not suitable to image a blocking shadow that eliminates pixel data along one side of a transition line. Blend plates cannot transfer mask patterns to create image blocking transitions between adjoining image tiles. Light mixing edge profiles cannot define contours to block patterns of pixels and thereby remove pixel overlap between adjoining image tiles. Blend plates are not designed to provide necessary mask resolution to minimize or eliminate duplicate pixels along a line or narrow zone of transition between adjoining image tiles.
In contrast to optical blending, blocking masks reduce or completely remove pixel overlap in regions of adjoining image tiles. This minimizes projection of duplicate pixel data. For a transition between two overlapping projection beams, each in a pair of blocking mask plates has an edge profile designed to prevent a portion of the duplicate pixel data in each of the two beams from impinging on the projection screen. Each blocking mask edge profile provides a contour which blocks light along a common line, e.g., in a narrow zone of transition between image tiles. On each side of the line or zone the screen receives pixel data from only one projection beam. By defining a transition line or zone in the region of overlap, each blocking mask removes pixel data from a different one of the beams on each side of the line or zone to eliminate projection of duplicate data onto the screen.
In the past, to accurately define the line or zone of transition within the region of tile overlap, edge profiles of blocking mask plates have been designed and fabricated based on, for example, the projection beam angles relative to the screen and the shape of the screen. In some cases the modeling can closely approximate necessary contours to remove brightness effects when, for example, the keystone effect is present on a cylindrically shaped screen (e.g., due to the fact that the projector may not have a lens shift and physically must be angled down/out of sight of the viewer, either overhead or underneath). Effective design of blocking mask plates becomes more challenging as projection systems incorporate more complex optical geometries and corrections to accommodate these geometries. To some extent optical distortions are correctable with software. Nonetheless, complex geometric effects can produce image tile projections which have tapered or nonlinear shapes. The shapes of the tile projections can be more complex when the screen has curvature in both horizontal and vertical directions.
With greater demands for higher levels of picture quality under conditions of low brightness levels, the known optical device designs and methods for creating seamless tile transitions either have intrinsic performance limitations or require greater cost due to increased system complexities. For example, when training and simulation systems display night scenes over water, the ability to resolve low light level information may be unsatisfactory, particularly when high brightness levels, due to duplicate pixel data, are present. These bright zones may persist when optical blending cannot sufficiently reduce light levels over a zone (e.g., by light scattering) or when blocking masks do not sufficiently follow a contour that substantially or completely prevents duplicate pixels from striking the screen.
Deficiencies in contours of blocking mask patterns may not be apparent until system installation, i.e., when bright zones become viewable on the screen. Then, to more completely remove bright zones caused by duplicate pixels, adjustments are made to the edge profiles of the blocking mask plates. Repetitive fabrication of the plates to optimize mask patterns adds significant time and expense. It can be a costly and difficult process to consistently remove excess brightness and create seamless transitions between image tiles under low light level conditions. There is a need for an improved apparatus and a method to define satisfactory patterns for light blocking along regions of tile overlap on the projection screen. Such an apparatus and method should reduce fabrication costs for blocking masks, reduce the time required for designing the masks, and reduce the time required to adjust the masks during and after system installation.